Hello.
Yes, you can use Grey V4 mixed with remains of Grey V3.
Explained: I had the following situation. It is known, that Formlabs recommend to use standard resin tank for 2 liters of resin. As soon as i have finished first liter on Grey V3, it was impossible to buy V3 due to it’s upgrade to V4. I asked Formlabs support if i can mix V3 and V4, and they confirmed that.

Pour remains of V3 somewhere to left resin tank out of it as much as you can. Do not use any cleaners of something like that to avoid damaging a tank. Small amount of V3 may left there. Put it back into printer.

Insert new V4 cartridge, do not forget to check it’s injection.

Reprogram your tank to V4. (do not remember the exact process, but there was nothing unusual there) Printer will notice you, that tank should be EMPTY.After starting a new print, tank will be filled with huge amount of new V4 resin. As it is new and empty. That’s why you should proceed step 1 carefully to avoid any issues.
Have already printed nearly 200 ml with such mix. No problem. I print small precise items on 25 microns mostly, and bigger parts with 50 microns resolution.
Good luck